The permalink or URL for the post contains the date on which it was published. If you change that date then the URL will be "broken" and if the post has been indexed by search engines what will happen when the URL is clicked is a 404 (page not found). Hence changing a date and breaking a URL is something to be avoided.
How to change the date is to edit the post and in the Publish box in the right column locate the date and click the "Edit" button, change the date and click "Update".
Example: Published on: Jun 7, 2010 @ 19:35 Edit
This entry on scheduling a post has good illustrations of the date/timestamp you can eyeball > http://en.support.wordpress.com/posts/schedule-a-post/
Lastly, note that when you edit a post and change the date/timestamp the post will not appear in the golabl tag pages on the "new" date. It's will appear under the tags you assigned and display in order of the date/timestamp. This software adjustment measure was introduced to stop those who were chronically editing their posts to keep them at the top of the global tags pages.